Thermochromic polymer layer and layer and method for production thereof

ABSTRACT

The invention relates to a thermochromic polymer layer and also a method for production thereof, The method is based on addition of a thermochromic colourant and if necessary further additives at the beginning of the extrusion process and subsequent extrusion to form the thermochromic polymer layer. Furthermore, the invention relates to a multilayer composite system comprising at least one thermochromic polymer layer and also at least one further layer.

The invention relates to a thermochromic polymer layer and also a methodfor production thereof. The method is based on extrusion with additionof a thermochromic colourant and if necessary further additives at thebeginning of the extrusion process and subsequent extrusion to form thethermochromic polymer layer. Furthermore, the invention relates to amultilayer composite system comprising at least one thermochromicpolymer layer and also at least one further layer.

Thermochromism comprises the property of a material to change its colourreversibly or irreversibly as a function of the temperature. This can beeffected both by changing the intensity and/or the wavelength maximum.Examples and theoretical backgrounds are described comprehensively inChromic Phenomena by Peter Bamfield (The Royal Society of Chemistry,2001) or in Thermochromic Polymers by A. Seeboth and D. Lötzsch(Encyclopaedia of Polymer Science and Technology, John Wiley & Sons,Inc. 2003).

The application for polymer flat films comprising polyethylene,polypropylene, polyester derivatives or a multiplicity of furtherpolymers and combinations thereof in multilayers is nowadays omnipresentin virtually all areas of life.

It is common to polymer materials with general thermochromic propertiesthat they generally have a thermally induced colour or transparencychange. Thus the colour change with simultaneous intensity reduction isdescribed in WO 02/08821. EP 1 084 860 claims the switching of thecolour effect, based on a donator-accepter colourant system, with atleast two further components. The change in colour is effected withinbroad temperature ranges, such as between −50° C. to 120° C. or −40° C.to 80° C. The entire colourant system can be inserted in the polymermatrix optionally also in the form of microcapsules with a diameter ofapprox. 50 μm.

The developments disclosed in U.S. Pat. No. 5,527,385 comprise inaddition organic hydrazide, sulphur or phosphorous compounds which areintended to improve for example the lack of light stability. The thermalstability is hereby negatively influenced.

The production of polymer thermochromic materials with the help ofprinting (laminating) of a thermochromic colour is a practical solutionfor some requirements and wishes of the packaging industry but does notachieve the set objective. Thus according to U.S. 2002037421, glassesare printed with a colour for use as sun protection or, corresponding toU.S. Pat. No. 4,121,010, polymers are coated with a thermochromiccolour, comprising sulphates, sulphides, arsenic, bismuth, zinc andother metals and oxides thereof. As a result, both the field of use isgreatly restricted and also as a result of the required additionalcoating (printing technique) cost-reducing continuous technology cannotbe used. In addition, it should be mentioned that the complexity ofprinting or labelling is often out of all proportion to the productioncosts.

The cited solutions in the mentioned patent specifications are notsuitable for producing large-area polymer films with thermochromicproperties. This applies also to the invention described in EP 1 157802, Here, during extrusion-blow moulding, thermochromic pigments areadded only in partial regions of the wall thickness of a plasticmaterial container. This takes place by adding a reversiblethermochromic pigment in the form of strip-shaped inserts. Thethermochromic pigments require in addition additional thermostablepigments and are added to or placed on the extrudate before leaving theextrusion head. Thorough mixing of thermochromic material and polymercarrier material is correspondingly not possible and is not sought hereeither.

Thermochromic pigments which are faded with a defined temperature effectand are commercially available are used here preferably. As is known,the pigments are added in the form of master batches in extrudertechnology. It has however been known for many years that master batchesbased on microencapsuled liquids, e.g. used in thermochromic inks, or onliquid crystal components, e.g. used in paints or films as heat sensors,do not have sufficient mechanical stability for use in extrudertechnology where the polymer melt is subjected to exceptional mechanicalrequirements at simultaneous increased temperature when passing throughthe extruder screw.

The object therefore underlying the invention is to produce a completelynovel polymer layer which has uniformly distributed inherentthermochromic properties over the entire layer.

This object is achieved by the method for producing single-layerthermochromic polymer layers having the features of claim 1 and also bythe thermochromic polymer layer having the features of claim 8. In claim12, a multilayer layer composite system based on a thermochromic polymerlayer according to the invention is claimed. The further dependentclaims display advantageous developments.

This object is achieved according to the invention in that already atthe beginning of the extrusion process, i.e. before passing through theextruder screw, a polymer material is added in addition which hasthermochromic properties and simultaneously is thermostable andresistant to substantially operating mechanical shear forces in thescrew process during melting of the master batch.

Preferably, the colourants are based on basic structures oftriphenylmethane colourants, pyridinium phenolate betaines,sulphophthalein structures, Reichardt colourants, thyranines, indicatorcolourants, azo pigments or fluoran colourants, such as e.g.2-chloro-6-diethylamino-3-methylfluoran.

As polymers, preferably polyethylene, polypropylene, polyester,polyamide and/or acrylonitrile-butadiyne-styrene-copolymer are used. Itis however also possible to use all the polymers currently used in theextrusion process.

As melting agents, preferably octadecanol, dodecanol, hydroxycarboxylicacids and/or 1-hexadecanol are used. There are possible as developerspreferably 2,2′-bis(4-hydroxyphenyl)propane,2,2′-bis(4-hydroxyphenyl)sulphone and gallic acid dodecyl ester.

The three components are used preferably in the weight ratio ofcolourant:melting agent: developer=1:94:5.

This process and the suitable embodiment of the extruder worm ensure therequired optimum homogeneous distribution of the thermochromic materialin the polymer layer. Preferably, there is thereby no concentrationgradient of the thermochromic material in the resulting layer so thatthe temperature-controlled colour is not a material-related, singularproperty of the layer, The polymer layer is inherently thermochromic.The thermochromic properties are not restricted to partial regions. Thisapplies explicitly also to this property with respect to thecross-section.

The choice of polymer materials which are used is only limitedtechnologically. All polymers used at present in the extrusion processcan be used. The thermochromic pigment based on a polymer material canalso be used without further polymer additives, such as for examplepolyethylene, polypropylene or polyester, i.e. as individual componentsfor producing a therrnochromic film. As melting agents and developersfor the thermochromic effects in cooperation with the pigments, allmaterials can be used according to the invention which fulfil thisfunction and which are destroyed neither thermally, mechanically norchemically during extrusion. Combinations of pigments, melting agentsand developers are possible.

The choice of polymer and its molecular mass influence the resultingthermochromic properties of the layer just as the molecular structure ofthe organic or inorganic pigment and the possibly used developers andmelting agents. The specific structure/property relationship of thepigments, melting agents, developers and polymer which were useddecisively determined the temperature region of the colour transitions.

According to the invention, the polymer layer can have a layer thicknessin the range of 1 μm to 10 cm. Preferably, the polymer layer has athickness of 1 μm to 1 mm. Preferably, the polymer layer is therebyformed as a polymer film.

It is evident to the person skilled in the art that, on the one hand,the technological parameters, such as the respective temperature in theworm housing, in the nozzle, on the surface of the chill-roll and alsothe flow speed in the extruder and the withdrawal speeds of thechill-roll and winding roll but also, on the other hand, the specificmaterial properties of the polymer and of the thermochromic pigmentsmust be coordinated to each other optimally.

The thermochromic monolayer with any arbitrary layer thickness can becombined with a further layer or even a plurality of layers formodification with additional properties. Multilayers of the generalarrangement, such as for example ABA or ABCBA, can be produced. Theindividual layers can hereby have the conventional function of aprotective or barrier layer , They can however also have thermochromicor non-thermochromic pigments of an organic or inorganic type in orderto produce further temperature-controlled colour effects. Correspondingto the additive or subtractive colour formation, the most varied ofcolour switches can be achieved within previously determined temperatureranges. The colour switches can be effected over a wide temperaturerange of ΔT 1-25° C. Preferably, the switch effect is effected innarrowly maintained temperature ranges of ΔT 1-2° C. Switches are alsopossible in the operating range of above 80° C., which is made possiblein particular by suitable choice of melting agent or combinationthereof. The temperature-induced change in intensity and in wavelengthmaximum can be complemented by additional translucence effects.

According to the invention, a multilayer layer composite system islikewise provided which contains at least one thermochromic polymerlayer, as is described above, and at least one further layer. There arepossible as further layer all the layers known in the state of the artwhich are suitable for extrusion.

With reference to the following FIGURE and examples, the invention isintended to be described in greater detail.

The FIGURE shows the schematic representation of an extruder known fromthe state of the art. This extruder has an extruder worm (a) on which asupply funnel (m) is disposed. Mixing of the thermochromic colourant canbe effected via this supply funnel. According to the state of the art,mixing has, in contrast to previously, been effected only via the feedblock with nozzle (c) shortly before leaving the extruder. The extrudedlayers are subsequently conducted over a cooling roller (e) and finallyrolled up to form a roll via the wind-up (g).

EXAMPLE 1

According to the invention, a thermochromic polymer film can be producedaccording to the following flat-film extrusion processes. The extrusionfunnel is supplied with a mixture of polyethylene (PE-LD) and a bluethermochromic pigment which is thermally and mechanically stable. Themixture ratio in % by weight is 94:6. The heating zones of the extruderare as follows: zone 1 at 175° C., zone 2 at 186° C. and zones 3, 4 and5 at respectively 194° C. The temperatures at the feed block and at thenozzle 1, 2 and 3 are 228° C., The speed of rotation of the worm is 68rpm. After several rninutes dwell time in the extruder, the polymer istaken up by the chill-roll at a speed of 6.3 m/min and a temperature of52° C. During winding up, the speed of the layer is kept constant. As aresult, a thermochrornic layer which is blue at room temperature isobtained, with a layer thickness of 39 μm which switches reversibly inthe temperature range of 38° C.-40° C. from blue to colourless, Thelayer is inherently thermochromic in its entirety.

EXAMPLE 2

According to the invention, a multilayer with thermochromic propertiescan be produced according to the following flat-film extrusion process.The extrusion funnel of the extruder C is supplied with a mixture ofpolyethylene (PE-LD) and a red thermochromic pigment which is thermallyand mechanically stable. The mixture ratio in % by weight is 86:14. Theextruder A for the protective layer is likewise supplied with PE-LD.Extruder B, responsible for the separating layer, is filled withpolypropylene. The heating zones of the extruder C are 180° C. 190° C.,195° C., 195° C. and 195° C. for zones 1 to 5. At the feed block and atthe nozzle the temperatures are 236° C. The speed of rotation of theworm is set at 58 rpm. At the extruder B, the temperatures for zones 1to 4 are fixed at 200° C., 210° C., 220° C. and 220° C. The worm runs at34 rpm. For extruder A, the temperature for the first heating zone is182° C. and for zones 2 to 5 is respectively 190° C. and the worm runsat 68 rpm. The chill-roll operates at 6.0 m/min at 55° C. As a result, athermochrornic ABCBA film which is red at room temperature is obtained,with a layer thickness of 135 μm. The thermochromic layer is hereby 45μm thick.

The two separating layers are respectively 20 μm and the two outerprotective layers 25 μm thick. The polymer film switches reversibly inthe temperature range of 83° C.-85° C. from red to colourless.

The technological parameters cited in the Examples, such as thetemperatures in the heating zones, the chill-roll, the speed of rotationin rpm of the worms, the withdrawal speed of the chill-roll in m/min andthe winding speed can be varied and combined in many different ways, ascan be detected immediately by the person skilled in the art.

EXAMPLE 3

According to the invention, a monolayer with thermochromic propertiescan be effected analogously to Example 1. A 50 mm slot die is herebyused and the chill-roll is not in operation. The speed of rotation ofthe worm is 93 rpm, As a result, a layer with a thickness of 0.28 cm isobtained.

1. A method for production of single-layer thermochromic polymer layersby means of extrusion in which at least one colourant, and optionally afurther additive or additives selected from the group consisting ofmelting agents and developers, is added to a polymer at the beginning ofthe extrusion process and is extruded to form the thermochromic polymerlayer.
 2. The method according to claim 1, wherein the polymer isselected from the group consisting of polyethylene, polypropylene,polyester, polyamide, and acrylonitrile-butadiene-styrene-copolymer andcombinations thereof.
 3. The method according to claim 1, wherein thecolourants are selected from the group consisting of pyridiniumphenolate betaines, sulphophthalein structures, Reichardt colourants,triphenylmethane colourants, pyranines, indicator colourants, flourancolourants, and azo pigments.
 4. The method according claim 1, whereinthe melting agent is selected from the group consisting of octadecanol,dodecanol, hydroxylic acids and 1-hexadecanol and combinations thereof.5. The method according to claim 1, wherein the developer is selectedfrom the group consisting of 2,2′-bis(4-hydroxyphenyl)propane,2,2′-bis(4-hydroxyphenyl)sulphone and gallic acid dodecyl ester andcombinations thereof.
 6. The method according to claim 1, wherein thecolourant is added to the polymer in a supply funnel of the extrusionprocess.
 7. The method according to claim 1, wherein the colourant, thepolymer and optionally further additives are provided in the form of amaster batch.
 8. A thermochromic polymer layer which can be producedaccording to the method according to claim
 1. 9. The polymer layeraccording to claim 8, wherein a reversible colour switching is effectedin a wide temperature range of ΔT from 1 to 25° C.
 10. The polymer layeraccording to claim 8, wherein a reversible colour switching is effectedin a narrow temperature range of ΔT from 1 to 2° C.
 11. The polymerlayer according to claim 8, wherein the colour switching is accompaniedby a changed translucence behaviour.
 12. The polymer layer according toclaim 8, wherein the layer has a layer thickness of 1 μm to 10 cm. 13.The polymer layer according to claim 12, wherein the layer thickness isfrom 1 μm to 1 mm.
 14. The polymer layer according to claim 12, whereinthe polymer layer is a polymer film.
 15. A multilayer layer compositesystem containing at least one thermochromic polymer layer according toclaim 8 and at least one further film.
 16. The method according to claim2, wherein the colourants are selected from the group consisting ofpyridinium phenolate betaines, sulphophthalein structures, Reichardtcolourants, triphenylmethane colourants, pyranines, indicatorcolourants, flouran colourants, and azo pigments.
 17. The methodaccording to claim 2, wherein the melting agent is selected from thegroup consisting of octadecanol, dodecanol, hydroxylic acids and1-hexadecanol and combinations thereof.
 18. The method according toclaim 2, wherein the developer is selected from the group consisting of2,2′-bis(4-hydroxyphenyl)propane, 2,2′-bis(4-hydroxyphenyl)sulphone andgallic acid dodecyl ester and combinations thereof.
 19. The methodaccording to claim 2, wherein the colourant is added to the polymer in asupply funnel of the extrusion process.
 20. The method according toclaim 2, wherein the colourant, the polymer and optionally furtheradditives are provided in the form of a master batch.